Blowholes in iron and steel are due to CO, H2, N2 gas. In rimmed steel, blowholes are mainly due to CO gas, because oxygen content in molten steel is much higher than its solubility in solid steel, and the pressure of CO gas in molten steel is high. Eut in semi-killed or killed steel, hydrogen or nitrogen gas becomes a factor of blowhole formation, because the pressure of CO gas in molten steel is very low and the solubilty difference of H2 or N2 gas between molten steel and solid steel is large.
As to blowholes caused by CO gas, the relation between formation of blowholes and Si, Mn, Al deoxidation was explained experimentally and the results of experiments were compared with calculation values from equilibrium constant (Report III). In this report, to study formation of blowholes by H2, N2 gas, H2 or N2 gas was lanced into molten steel, and the relation between blowhole formation by H2, N2 gas and deoxidation was explained.
Blowhole samples were melted by a 100kg basic high frequency electric furnace similarly to Report III. When the temperature of molten steel attained to 1600°C, deoxidation reagents (Si, Al) were added in different quantities. Some time after deoxidation, H2 or N2 gas was lanced into molten steel for a time. 25kg of molten steel was immediately tapped to a ladle and then poured to a 16kg ingot. In succession four ingots were cast, similarly.
The results obtained from this experiments were as follows.
1) Blowholes caused by hydrogen
a) Si deoxidation...As hydrogen content increased, the limit of blowhole formation increased to 0.2% Si from 0.1% Si. But when Si content was more than 0.2%, blowhole by hydrogen gas did never grow under 80×10-5% H2.
b) Al deoxidation...When Al content was more than 0.01%, blowhole by hydrogen gas did not grow under 80×10-5% H2. Accordingly when H2 content was less than 80×10-5% H2, hydrogen was not the main cause to blowhole formation if the molten steel was deoxidized fully.
2) Blowholes caused by nitrogen
a) Si deoxidation...As N2 content iacreased, the limit of blowhole formation increased to 0.2% Si from 0.1% Si. But when Si content was more than 0.2%, blowhole by N2 gas did never grow under 100×10-4% N2.
b) Al deoxidation…As N2 content increased, the limit of blowhole formation increased to 0.08% Al from 0.01% Al. Accordingly, in case of Al deoxidation N2 gas probably became the main cause of blowhole formation.